1. Field of the Invention
The present invention relates to color data processing in full-color printing equipment and related equipment such as printers, video printers, and scanners, or in monitors and other display equipment. More particularly, the invention relates to a color data conversion apparatus and method for image output equipment that displays images by using four or more primary colors.
2. Description of the Related Art
Conventional color image display apparatus is generally trichromatic, operating with the three primary colors red, green, and blue. In an XYZ color coordinate system, the gamut of colors that can be displayed is represented by a triangle in the XY plane (referred to as an XY chromaticity diagram), the vertices of the triangle being the three primary colors. To enlarge the gamut of colors that can be displayed, the color coordinates of the vertices are preferably chosen so as to enlarge the area of the triangle. This can be done by attenuating unnecessary wavelength components of the primaries, so that their spectral curves are concentrated around a single wavelength, but in many types of display apparatus this leads to problems such as reduced brightness or increased power consumption. The shape of the spectrum locus on the XY chromaticity diagram also suggests that the trichromatic color expression is not necessarily efficient in increasing the gamut of displayable colors.
Recent research and development work is therefore shifting toward apparatus that uses four or more primary colors. The gamut of colors that can be reproduced by such apparatus is represented in the XY chromaticity diagram by a polygon with four or more vertices. Arbitrary colors within the polygon are generated from data with four or more values that control, for example, the emission brightness (in apparatus of the self-emitting type) or transmittance (in liquid crystal display apparatus) of the primary colors.
An essential requirement for such apparatus is a device or method for converting trichromatic image data to image data expressed in terms of the four or more primaries, not only to maintain compatibility with existing apparatus but also for reasons such as data size, and because most color image capture devices generate trichromatic color data.
Color conversion is already a known art. One known method, disclosed in Japanese Patent No. 3432468, operates on trichromatic image data to obtain chromatic component data for six colors (red, green, blue, cyan, magenta, and yellow), then performs further operations on the chromatic component data to obtain polynomial data with eighteen terms, selects valid (non-zero) terms, and performs a matrix operation on the valid terms and the three primary chromatic components to obtain trichromatic output data. This method can be implemented as a color conversion lookup table stored in a read-only memory (ROM).
In principle, a similar method could be used convert trichromatic image data to image data with four or more primary colors, by storing four or more output data values in the ROM for each combination of three input values. Besides requiring additional memory space, however, such a method would lack flexibility, and in any case the above disclosure does not explain how four or more output values should be derived. A particular problem is that with four or more primary colors, the same color can generally be expressed in many different ways, and it is not clear how to select one of those ways.